(1) Field of the Invention
This invention relates to a hydraulic drive circuit system, and particularly to a hydraulic drive circuit system suitable for a construction machine, such as a wheel loader or tractor, or a like apparatus in which at least two hydraulic pumps are commonly driven by a single engine, with the pumps performing different operations respectively.
(2) Description of the Related Art
A hydraulic drive circuit system of a wheel loader or tractor is generally provided with separate hydraulic pumps, one for driving the wheels and the other for a front attachment, and both hydraulic pumps are commonly driven by a single engine. The hydraulic pump for the wheels is of the variable displacement type, and is controlled in such a way that its revolution number and its delivery rate per revolution are both increased as the revolution number of the engine increases in accordance with the stroke of an accelerator pedal.
In a hydraulic machine in which such a control is performed, the running speed of the machine naturally increases when the accelerator pedal is depressed to increase the revolution number of the engine in order to increase the revolutionary speed of an actuator for a front attachment (for example, to increase the working speed of a bucket). In other words, the correlation between the working speed of the front attachment and the running speed is substantially fixed. Because of this combination of working speed and running speed, the matching of the two may become unsuitable, depending on the type of work, whereby work efficiency could be impaired.
With the foregoing in view, a hydraulic drive circuit system designed to decrease the running speed while at the same time increasing the working speed has been proposed for wheel loaders or tractors. This hydraulic drive circuit system is provided with a pedal-operated control means called an "inching pedal" in addition to an accelerator pedal, whereby deceleration of the running speed is also permitted by the control means.
The above hydraulic drive circuit system is equipped with a hydraulic actuator for actuating a displacement varying mechanism, for example, a swash plate of a hydraulic pump for wheels, a working fluid feeding circuit for feeding a working fluid of a pressure produced responsive to the revolution number of an engine to the hydraulic actuator, and an inching valve connected to a communicating line interposed between the working fluid feeding circuit and a low pressure source, the opening of said inching valve being variable to control the degree of actuation of the hydraulic actuator. The inching valve is operated by the inching pedal mentioned above. The working fluid feeding circuit is composed, for example, of a charge pump driven by the engine to feed a charge pressure to a closed circuit formed of the hydraulic pump for the wheels, a restrictor provided with a delivery line from the charging pump, and a pair of lines connecting both the upstream and the downstream side of the restrictor to the hydraulic actuator. In this case, the above-mentioned communicating line is interposed between the paired lines. Also connected to the paired lines is a directional control valve as a working direction setting means for controlling the operating direction of the hydraulic actuator.
When the accelerator pedal is depressed in such a system, the revolution number of the engine increases and the delivery rate of the hydraulic pump for the front attachment also increases. The working speed of the front attachment hence becomes faster. At the same time, the revolution number of the hydraulic pump for the wheels also increases. If the directional control valve has been changed over to the forward or reverse position from the neutral position, a differential pressure occurs between the upstream side and downstream side of the restrictor with respect to the working fluid delivered from the charge pump acts on the hydraulic actuator by way of the paired lines, so that the hydraulic actuator is operated. The tilting of the swash plate hence increases, so that the delivery rate per revolution of the hydraulic pump for the wheels increases. As a consequence, the delivery rate of the hydraulic pump increases in a quadratic curve as a function of the revolution number and the tilting of the swash plate. When the operation of the hydraulic actuator reaches the maximum degree (namely, when the tilting of the swash plate reaches the maximum), the delivery rate per revolution does not increase any further, and the delivery rate of the hydraulic pump changes as a function of only the revolution number of the engine and thus increases linearly.
When the running speed has increased beyond necessity in a depressed state of the accelerator pedal and its matching with the work by the front attachment has been impaired, the inching pedal which is normally closed is depressed to operate the inching valve. As a result, the inching valve is opened in proportion to the degree of the depression of the inching pedal; the working fluid on the higher pressure side of the paired lines is allowed to flow to the lower pressure side, and the tilting of the swash plate is therefore reduced in order to slow down the running speed. It is accordingly possible to stop the vehicle or to slow it down to a desired speed by controlling the degree of operation of the hydraulic actuator through an operation of the inching pedal in the above-described manner.
As has been described above, the conventional system requires an operator to depress not only the accelerator pedal to control the revolution speed of the engine, but also the inching pedal to control the running speed so that the most suitable matching point is found between the working speed and running speed. He is required to perform the work in this state. However, this manner of operation requires depressing the accelerator pedal and the inching pedal at the same time, and also controlling the strokes in a correlated relation. The operator is therefore hard put to maintain his body's stability, and is tired unnecessarily.
The operator is also required to control a working lever to operate the front attachment or the like in the above state. It is hence difficult to perform delicate work. If his foot should slip off the inching pedal in the course of work, the vehicle runs at an unexpected speed. This is very dangerous. Even when the vehicle is stopped and work is performed at a constant engine revolution number, the operator is required to continuously depress the accelerator pedal and inching pedal; his work load is not reduced at all.
With a view toward easily enabling low-speed running suited for combined operations without posing an extra load on the operator, the present applicant has already proposed in U.S. patent application Ser. No. 07/127,275 to provide an accelerator lever as a means for setting a target revolution number for an engine, a means for guiding a working fluid from a working fluid feeding means to a low-pressure circuit in relation to the stroke of the accelerator lever so as to limit the maximum pressure of the working fluid, and a means for selectively actuating the operation of the maximum pressure limiting means in a hydraulic drive circuit system of the above-described sort.
Once the maximum pressure limiting means is activated in the proposed system, the pressure increase of the working fluid from the working fluid feeding means is limited; for example, when the accelerator lever exceeds a predetermined stroke. Even when the stroke of the accelerator lever is further increased, the degree of actuation of the hydraulic actuator does not increase, and the tilting of the swash plate does not increase correspondingly. The delivery rate of the hydraulic pump therefore increases only when the revolution number of the engine increases. After the accelerator lever has reached the predetermined stroke, the increment of the delivery rate of the hydraulic pump decreases, hence reducing the increment of the running speed. Control of the acceleration lever alone therefore allows the operator to govern the running speed within a reduced speed range and the working speed in accelerated speed ranges.
However, the proposed system cannot achieve high-speed running when the maximum pressure setting means has been actuated to set the running speed in a low speed mode. Since the running speed is controlled by the accelerator lever and by changing at least the revolution number of the engine, the control of the accelerator lever also changes the working speed of the front attachment as a result of the change in the engine's revolution. It is hence impossible to control the running speed independently of the working speed of the front attachment. As a consequence, the proposed system cannot perform work in which the control of the running speed is required in a wide speed range of from a low speed to a high speed concurrently with, before, or after the operation of the attachment; or in which the running speed has to be controlled independently of the working speed of the front attachment in the combined operations of both front attachment and running.
For example, work by a hydraulic machine such as a wheel loader or tractor includes operations where digging is performed by utilizing the running function, and then the dug-up earth is carried away to another place. In such operations, it is convenient from the standpoint of work ease to drive the front attachment under high power or at a high speed while fine-adjusting the running speed at low speeds. As for work efficiency, it is more convenient to perform the carrying of the earth at a running speed as high as feasible. While conducting such an operation, it is possible to drive the front attachment under high power or at a high speed, and to control the running speed at a low speed, so long as the maximum pressure limiting means is actuated as mentioned above. The carrying of the earth cannot be performed at a sufficiently high speed after the digging, since the running speed is set in the low-speed mode. Since the control of the running speed also changes the working speed of the front attachment in the combined operations of the running and the front attachment, it is still impossible to control the running speed at will and hence difficult to ensure good operability.